1. Field of the Invention
The present invention relates to a non-destructive method and apparatus for testing adhesive quality of a metallic coating on metallic substrates formed in either a flat or cylindrical specimen. In particular, the present invention relates to testing of metal coatings on the inside of a gun bore and evaluation of the adhesion quality of such coatings.
2. Description of the Prior Art
Coatings serve several functions; one is to extend the life of the component. After all precautions have been taken in manufacturing, failure of components is frequently initiated at the surface, and this is particularly true in an aggressive environment. Some coatings are deposited to protect the surface of the substrate. The coating protects against corrosion, fatigue, temperature, and erosive effects. Any coating can either fail by loss of adhesion.
The problem of evaluating metallic coatings has always existed. Chromium coatings on gun tubes have been used for fifty years. At present there is no quantitative means for the evaluation of metallic coatings. The scratch test uses a smoothly rounded stylus to scratch the surface of the film as the load is gradually increased. The load at which the film under the stylus is detached and a clean scratch is created, has been used in the past to indicate the xe2x80x9cqualityxe2x80x9d of the coating. It is a comparative-type test. This test is destructive and hence not applicable to a part in use. Presently, there does not exist a non-destructive way of evaluating a coating, and especially of evaluating a coating which is not readily accessible. For example chromium has been plated on the bore of gun tubes for over fifty years, and the way of evaluating the coating has been to actually fire the gun and visually observe the amount of coating not adhering to the bore. Previously, the coating had to be destroyed for it to be evaluated. This evaluation has included visual, qualitative, and comparative analysis.
Ultrasonic inspection is one of the most important nondestructive techniques for inspecting materials and structures. Conventional ultrasonic inspection suffers from two important limitations: first, there is need of contact between the transducer and the inspected part and most often need of coupling fluid bath or fluid column (such as water) to transmit ultrasound and secondly the transducer should be properly oriented with respect to the surface when single side inspection is performed (operation in reflection or pulse echo mode). Thus, inspection of samples at elevated temperature or complex geometry is difficult. Such techniques cannot readily be used in a preferred use of the invention for examining layered metal coatings within a gun bore.
Such limitations are circumvented by laser ultrasonics, an ultrasonic inspection technique, which uses lasers to generate and detect ultrasound. For generation, a high power short pulse laser is generally used and the ultrasonic waves are produced by the surface stresses induced by the heat generated by laser absorption or by the recoil effect following surface ablation. For detection, a continuous wave or long pulse laser is used in association with a Michelson interferometer which is sensitive to the ultrasonic surface motion and gives a signal representative of this motion.
U.S. Pat. No. 5,724,138 teaches a laser ultrasonics technique that characterizes a composite dispersive response signal from a semiconductor wafer under analysis for temperature detection during processing of the wafer. An entire dispersive response signal is analyzed by using discrete wavelet transform analysis. However, this technique does not teach or suggest apparatus or method, which determines quantitatively adhesion quality of a metal coating on a substrate, or mechanisms for doing such evaluations where the examined structure has a cylindrical surface such as a gun barrel. Thus, there is need for an integrated approach for examining metal-coated surfaces as to their adhesive qualities using laser ultrasonic apparatus.
The present invention is a nondestructive and quantitative laser ultrasonic apparatus and associated method for determining adhesion quality of a coating on a substrate. The apparatus of the invention is preferably a Michelson-type interferometer based system and includes a rotary probe head assembly for making evaluations within a cylindrical test specimen. The method of the invention can be used when the coating acoustic impedance is equal, greater or less than the substrate, and the coating layer is thin, i.e. situations where conventional ultrasonic techniques are not effective. The method of the invention includes data analysis that uses acquired data from the ultrasonic laser apparatus and computes the dispersion relation (frequency versus velocity) and from this, outputs the adhesive quality of the coating. In the analysis, a layer which has xe2x80x9cgoodxe2x80x9d adhesion to the substrate is attached by a xe2x80x9cweldedxe2x80x9d bond and a layer which has xe2x80x9cpoorxe2x80x9d adhesion attached by a xe2x80x9csmoothxe2x80x9d bond. In the analysis, the difference between these two cases is defined by the boundary conditions imposed on the coating layer and the substrate. The result of the analysis of the xe2x80x9cweldedxe2x80x9d and xe2x80x9csmoothxe2x80x9d bonds of a test specimen shows a difference in the shapes of the respective dispersion curves. In wavelet analysis, differentiation between xe2x80x9cgoodxe2x80x9d and xe2x80x9cpoorxe2x80x9d adhesion bonds is qualified and quantified using a ridge-following technique.
Accordingly, advantages of the invention using a laser ultrasonic apparatus and method include:
a) A system that enables rapid measurements at remote locations which are accessible to optical fibers, or direct laser beam incidence;
b) A system that can be used in industrial environments and does not require physical contact with the test specimen, which can have flat or cylindrical surfaces directly accessible to these laser beams; and
c) A system and method that lends itself to automation such that the test specimen can be scanned efficiently by ultrasonic laser hardware and analyzed efficiently using a ridge-following technique in wavelet analysis by comparing theoretical results with analyzed experimental results of the test specimen.
Still further advantages will become apparent from consideration of the ensuing detailed description.